The present invention generally relates to a photographic camera having a two-curtain type focal plane shutter and, more particularly, to a photographic camera having an exposure control system which, when used in combination with an electronic flash device, is switched from an automatic daylight exposure mode to an automatic flash exposure mode when a predetermined voltage level is reached in the flash device.
The U.S. Pat. No. 4,016,575, patented on Apr. 5, 1977 or its corresponding German Offenlegungsschrift Nr. 25 14 535 published on Oct. 30, 1975 discloses a flash photography exposure control system for a photographic camera having an electrically timed shutter provided with an electronic or mechanical changeover switch by which the camera is switchable from an automatic daylight exposure mode to an automatic flash exposure mode when a threshold voltage level is reached in the electronic flash circuitry of a strobe or flash device detachably coupled to the camera.
This known flash photography exposure control system comprises a first timing circuit which is rendered operative when the camera is set in the automatic daylight exposure mode, a second timing circuit which is rendered operative when the camera is set in the automatic flash exposure mode, a comparator selectively responsive to the first and second timing circuits for energizing and deenergizing an electromagnetic coil for exposure control, and a changeover switch responsive to the voltage of a trigger capacitor in the flash device for selectively connecting the timing circuits to the comparator.
The second timing circuit includes a variable resistor forming a part of the time constant circuit together with a capacitor included in the first timing circuit, which variable resistor is so adjusted as to provide a timed interval for flash photography.
With the known flash photography exposure control system, a photographer trying to take a photographic picture of a target object to be photographed under flash lighting need not adjust the shutter speed selecting dial to a predetermined shutter speed, for example, 1/60 second or less, which had theretofore been required because of a compromise between the large inertia of the shutter and the output characteristics of the flash device. This is possible because, when the voltage stored on a capacitor in the flash device, this voltage being discharged through the discharge flash-tube upon actuation of a trigger circuit including a trigger capacitor to illuminate the target object, attains a predetermined amount, the first timing circuit becomes inoperative while the second timing circuit becomes operative. In other words, the known flash photography exposure control system is designed so that the changeover switch causes the first timing circuit to be inoperative and the second timing circuit to be operative when a threshold voltage level is reached in the electronic flash device.
While the known flash photography exposure control system is satisfactory, it has been found that the shutter remains opened for an appreciable period of time subsequent to the firing of the flash device. This is because the electromagnetic coil operatively associated with the shutter is connected to the second timing circuit during the actual taking of a photographic picture under flash lighting so that the electromagnetic coil is deenergized to close the shutter only after a predetermined period of time determined by the setting of the variable resistor of the second timing circuit has passed.
The fact that the shutter remains opened for an appreciable period of time subsequent to the firing of the flash device may be undesirable depending upon the actual situation in which the photographic picture is taken. Moreover, the employment of a variable resistor for controlling the shutter speed for flash photography appears to cause variation in the quality or performance of the flash photography exposure control systems in view of the fact that the manufacture of the variable resistors having uniform and equal performance characteristics is difficult.